1. Field of the Invention
The present invention relates to solid imaging devices and methods of manufacturing the same, and particularly relates to structures and methods of forming substrate voltage generating circuits used for controlling blooming voltages of solid imaging devices.
2. Background Art
FIG. 9 illustrates an example of conventional CCD-type solid imaging devices. In this example, as shown in FIG. 9, the photoelectric conversion portion 70 comprises a P-type well 72 in an N-type substrate 71, and an N-type region 73 is formed on the P-type well 72. The charge transfer portion 74 comprises a charge transfer electrode 75 covered by a light shielding film. Electrons are transferred to the N-type region 78 of the charge transfer portion 74 after being stored in the N-type region 73. If the amount of the charge stored in the N-type region 73 of the photo conversion region 70 exceeds a transferable charge quantity, the charge overflows from the region during the transfer operation. Thus, the CCD-type solid imaging device is designed such that the charge exceeding the necessary amount is swept to the substrate so as not to exceed the transferable amount.
As shown in charge distribution diagrams of FIGS. 10A and 10B, the amount of charge stored in the photoelectric conversion region is determined by the potential barrier .PHI.PW of the P-type well region which constitutes the vertical overflow drain structure (VOD). That is, when the generated charge exceeds the amount of the charge which can be stored in the N-type region, the charge exceeding the storable amount is swept to the N-type semiconductor substrate by going beyond the potential barrier .PHI.PW of the VOD. The amount of the charge which can be stored in the photoelectric conversion region, in other words, the height of the potential barrier .PHI.PW, can be controlled by the substrate voltage V.sub.sub, which is the voltage applied to the substrate which constitutes the drain (this substrate voltage is called the blooming control voltage).
The potential distribution curve fluctuates as shown by the solid line or the broken line of FIG. 10A, caused by fluctuations of the impurity in the concentration or the depth at a wafer surface due to fluctuations of the impurity profile at the time of ion implantation in the manufacturing process, and the height of the potential barrier .PHI.PW also fluctuates for each device having different values such as .PHI.PW.sub.1 or .PHI.PW.sub.2. As a result, since the device characteristic fluctuates due to the fluctuation of the amount of the charge which can be stored in the photoelectric conversion region, the amount of charge of the conventional device has been controlled so as to be constant by changing the blooming control voltages V.sub.sub and V.sub.sub by setting for every device different substrate voltages applied from the circuit side of the camera system.
However, when in use, in the case of applying a different voltage for each device from the circuit side of the camera system, the circuit structure for generating different substrate voltages at the camera side becomes complicated, and the manufacturing line also becomes complicated because it becomes necessary to prepare different components for each device. Accordingly, customers come to require providing in the solid imaging device a substrate voltage control circuit which controls the substrate voltage so as to conform to the individual imaging device. A substrate voltage control circuit is in practical use, which, for example, produces a desired voltage by a resistance dividing method for obtaining an optional contact among a plurality of resistors which are connected between the source potential and the earth potential. An example of the substrate voltage control circuit is proposed, which is constructed by use of a MOSFET, fuses, and source followers (Hirouki Yamauchi et al., "A Ultra Small Sized 1 mm 50,000 Pixels IT-Image Sensor", Image Information Media Academy, Mar. 27, 1998). Another example using a non-volatile memory transistor is reported, and a patent application filed by the present inventor (Japanese Patent Applications, First Publications No. Hei 10-84112, and No. Hei 10-15737).
In the CCD-type solid imaging device, malfunction is sometimes caused by contamination with dust. In general semiconductor devices such as memory devices, the wiring layer is covered by the passivation layer so that inconvenience will not be resulted even if the passivation layer is contaminated by dust. However, in the CCD-type solid imaging device, if the photoelectric conversion region is contaminated by dust, the dust particle blocks the light incident to the photoelectric conversion region and the pixel covered by the dust particle will form a black defect. A problem arises that the black defect or so called black flaw degrades the quality of the display. In other words, more attention should be taken to prevent the CCD-type solid imaging devices from being contaminated by dust than the other semiconductor devices such as memory devices.
However, when a substrate voltage generating device uses fuses, it is necessary to cut the fuse for controlling the substrate voltage. For cutting the fuse, application of a high voltage or irradiation by laser light are carried out, which causes dust by scattering metal particles constituting the fuse. Therefore, it is not desirable to install a highly volatile fuse in the CCD-type solid imaging device, in order to prevent from generating black flaws.
When the substrate voltage control circuit uses a non-volatile memory transistor, a problem arises that a threshold voltage of the transistor changes when light (especially, ultraviolet light) is incident to the transistor, and reliability of the substrate voltage generating circuit is degraded because its characteristic fluctuation becomes great.
In such a circumstance, it has been desired to provide a CCD-type solid imaging device which does not generate an inconvenience in the display quality and which is provided with a highly reliable substrate voltage control circuit. It has been desired also from the points of view of high integration of the solid imaging device and of low cost production to provide a substrate voltage control circuit having the smallest occupied area and capable of being produced by comparatively simple manufacturing process.